For purposes of this disclosure the term "crane" shall refer to any hoisting equipment designed to pick up a load, transport it a limited distance, and deposit it at a new location. The term "latticework construction" refers to structural crane members of open truss design including four chords and interconnected three-dimensional lacing extending across and diagonally between the chords. While the novel latticework construction shall be described in relation to the upright boom on a mobile crane, it is to be understood that it is also applicable to other common crane components, including (but not limited to) auxiliary booms or jigs, towers, masts, gantries, derricks and gin poles.
Load supporting cranes are commonly used in building and other construction for moving very heavy objects. Such cranes commonly employ a boom assembly which is pivotally mounted at one end to a stationary or mobile base support. Suitable cable rigging is employed to alter the inclination of the boom assembly with respect to the ground in order to perform the various functions for which the crane was made. The construction and crane industry in recent years has employed increasingly longer and larger boom assemblies in order to meet the requirements and demands of today's construction and excavation needs.
Such large boom assemblies are generally constructed in a plurality of disconnectable segments to permit dismantling for transportation between construction sites. The boom segments are made of a plurality of longitudinal chord members interconnected by a plurality of lacing members to provide the desired structural strength. Terminal end portions of each chord member are typically provided with suitable connections for securing abutting boom segments together.
The outermost, or tip, boom segment typically includes a weldment assembly for movably supporting hoist cables which carry a load block for engaging the various items to be moved by the crane. It is generally important that the area between the front chord members be open along the length of the tip boom segment to enable the hoist cables to enter within the cross-sectional area of the tip segment when the boom assembly is pivoted to its maximum upward position. By providing reaction forces to the load within the cross-sectional area of the outermost boom segment in such condition, the loads on the boom are better distributed among all four chord members in each of the segments. If the front area across the boom section were closed, it would be necessary to project the hoist cable carrying weldment assembly forwardly to prevent the cables from bearing against lacing members of the boom tip segment when the assembly is pivoted to its maximum upwardly position (80.degree.). However, this projection would exert greatly unbalanced forces on the front and rear chords along the boom.
These concepts of crane and boom assembly construction are, for example, illustrated in U.S. Pat. Nos. 3,249,238 to Hedeen; 3,511,388 to Markwardt; 3,955,684 to Novotny; 4,394,911 to Wittman et al.; 4,537,317 to Jensen; and 4,621,742 to Rathi. Such cranes, however, are not without drawbacks. To provide this open front area in the outermost boom segment, an alternate way of providing support between the pair of front chord members must be provided. For example, intersecting lacing members generally extend between each of the two side and rear pairs of chord members to rigidify the segment. However, were such lacing members to extend between the front chord members, the hoist cables would not be permitted to enter into the cross sectional volume of the outermost boom segment when raised to its most upwardly position.
The Hedeen U.S. Pat. No. 3,249,238 teaches overcoming this problem by employing additional longitudinal front chord members which outwardly expand to provide an open area for receiving the hoist cables along the substantial length of the front of the boom segment. Other elaborate rigidifying connections are typically employed which extend between and interconnect the front pair of chord members while providing an open space for enabling the hoist cables to extend through the cross sectional volume. However, all such interconnections tend to diminish the structural load carrying capacity of the outermost boom tip segment (and correspondingly the entire boom assembly) below that which would be present if lacing members extended between the front chord members in the same manner they extend between the rear and side chord member pairs.
A further separate drawback with typical large cranes includes difficulties in their transport between construction sites. Even though adjacent boom segments are separable from one another, enabling the length of the boom assembly to be reduced for ease of transport, the cross sectional size of each individual boom segment can become too large for transport on conventional highways. For example, the width and height span of individual boom segments in large cranes can exceed fourteen feet. Placing such segments on trailer beds for transportation along a highway produces a vehicle having a height and width which exceed the maximum allowable for most highways and bridges.
Yet another separate problem relating to existing cranes involves built-in pre-stress that occurs during manufacture. For example, many boom assemblies, and each of the boom segments of a multi-segmented boom assembly, have the lacing and chord members interconnected by welding. As each such boom or boom segment is constructed, internal or residual stresses are inherently built into the finished structure when the various members are positioned and welded. These internal stresses diminish the load carrying capacity of the boom or boom segment that would otherwise be attainable were it practical to construct a stress-free boom or boom segment of the same size. These internal stresses also add to the cyclic moment loading that occurs at each connection weld point and can contribute to fatigue and premature failure of the weld. Because of this diminished load carry capacity, booms or boom segments must be manufactured to have a larger cross sectional span for a given load capacity than would be necessary were no pre-stress present. Accordingly, this problem compounds the size requirement of the boom or boom segment, further aggravating the transportation problems associated with large cranes.
Concepts employed by the present invention enable drawbacks such as these to overcome.